Azimuthal sonic measurements are currently made commercially by major service providers in the wireline domain in the form of crossed-dipole shear anisotropy. Because wireline tool do not rotate quickly in the well bore (they typically rotate once every few minutes, not multiple times per second as in the case of logging-while-drilling (“LWD”) tools), they cannot easily acquire data at many azimuths.
Existing wireline systems use a crossed-dipole tool, which is a tool with a dipole source firing in the x-direction and a second dipole source firing in the y-direction. Typically, x- and y- are not acquired simultaneously, but as close as can be without the signals overlapping yet still being considered to be at the same depth. There are typically arrays of receivers located on the x- and y-axis. The signal from the x dipole source is recorded on the x receivers and y receivers, these datasets being labeled XX and XY respectively. The signal from the y dipole source is recorded on the x receivers and y receivers, these datasets being labeled YX and YY respectively. Through Alford rotation, waveform inversion, or a combination of various techniques, and accounting for tool centralization, source and receiver matching, and a circular borehole, an estimated predicted waveform set at each angle around the well bore can be computed from the 4 sets of acquired waveforms. Various computational methods can then be employed to determine the maximum and minimum shear slowness and the angle of the anisotropy. The waveforms at angles other than the 4 sets measured are inferred or estimated and may not be directly measured.
In these methods if the tool is oriented in line with the anisotropic field, the tool would see no variation on the crossline axis, and the anisotropy would be missed. In addition, these methods might not be as sensitive in complex anisotropic regimes where there is depth-of-investigation variation in the flexural mode response. It is also challenging to acquire a good flexural mode response and separate it from the Stoneley wave. In addition, large errors can occur in anisotropy calculations, and indeed trying to measure anisotropy at all, with a wireline tool in a horizontal hole where the tool is off-centered (e.g., lying on the bottom of the hole).
Existing systems use wireline crossed-dipole tool design.